Thanks for the suggestions. Galvanized iron pipe sounds good, but I wouldn't have a good way to cut it to length. I think I'll go with 1x4s, attached with galvanized nails. Seems easier to grab onto when climbing than 2x4s.. If I'm feeling it, I will cut slots in the rails so 1x4s are flush. There will be a battle in my head between my desire to do this quick and easy and my inherent anal-retentive nature ;-)